1. Field of the Invention
The present invention relates generally to a process of producing a capacitor in a hole of a silicon substrate.
2. Description of the Related Art
European Patent Application EP 0 528 281 discloses a silicon capacitor. This comprises an n-doped silicon substrate whose surface is structured in a characteristic way by an electrochemical etching in a fluoride-containing, acidic electrolyte in which the substrate is connected as an anode. In the electrochemical etching, more or less regularly arranged hole structures form at the surface of the substrate. The hole structures have an aspect ratio up into the region of 1:1000. The surface of the hole structures is provided with a dielectric layer and a conductive layer. The conductive layer, dielectric layer and silicon substrate form a capacitor in which, owing to the increase in surface brought about by the hole structures, specific capacitances of up to 100 .mu.FV/mm.sup.3 are achieved. In order to increase the conductivity of the substrate, it is proposed to provide an n.sup.+ -doped zone at the surface of the hole structures.
Normally, silicon capacitors are produced in silicon wafers. In this process, a bending of the silicon wafers is detected which is associated with mechanical strains due to the n.sup.+ -doped zone at the surface of the hole structures, which are up to 300 .mu.m deep. This bending of the silicon wafer results in problems in further process steps such as lithography, reduction in wafer thickness and chip separation, which are necessary for incorporating the silicon capacitor in a package.
The publication by A. Fukuhara et al., J. Appl. Cryst. (1980), vol. 13, pages 31 to 33 discloses a study of the compensation for mechanical strains in silicon crystals. A strain is observed which is essentially proportional to the dopant concentration and which can be compensated for by an additional doping with germanium. Layers 1 to 5 .mu.m deep are doped with germanium and/or boron. The germanium is introduced by diffusion, an annealing time of 14 days being necessary at a temperature of 1473 degrees K.
The publication by A. Heuberger, Mikromechanik, Springer-Verlag 1989, pages 216-236 discloses that highly boron-doped silicon layers which are used as etch-stop layers in micromechanics and which are grown epitaxially on silicon substrates cause bends in substrates which are compensated for by additionally introducing, for example, germanium into the boron-doped layer.